Objective: To investigate the role of C-type viruses in human cancers. Approach: Fibroblast cell strains from normal newborns and from patients with leukemia and diseases which often predispose patients to malignancies (Down's syndrome, ataxia telangectemia, Wiscott-Aldrich) will be tested for the induction of latent C-type virus. Diploid cell strains will be treated with agents known to induce C-type virus; i.e., 2-deoxy-D-glucose, puromycin, cyclohexamide, iododeoxyuridine and long term in vitro cultivation. Following exposure to these agents, the cell culture fluids will be examined for the presence of particles with density 1.16-1.18 g/ml which incorporate uridine, contain reverse transcriptase activity and have typical viral morphology by electron microscopic analysis. Cells will be examined before and following exposure to inducing agents for the presence of the 30,000 dalton interspecies specific antigen of the mammalian C-type viruses. We will observe whether induction is correlated to antigen expression prior to treatment with inducing agents and the clinical syndrome of the patient whose cells are tested.